1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a storage medium for reducing noise in image data of a captured image.
2. Description of the Related Art
There is a demand that a camera have higher sensitivity than hitherto in order to capture an image of a darker scene or a faster object. However, an increase in the sensitivity of the camera causes an increase in noise, and therefore, noise reduction processing for more greatly reducing the noise becomes necessary. Simple noise reduction processing causes a deterioration in resolution, and therefore, a technology for reducing noise while retaining an edge or a texture is necessary. A general approach for such a noise reduction technology of retaining the edge involves determining similarity between a target pixel, to be reduced in noise and a reference pixel around the target pixel, determining a weight according so the similarity, and calculating a weighted mean, thereby reducing noise. For example, a technology called bilateral filtering determines a weight according to the similarity calculated based on a difference in pixel value between the target pixel and the reference pixel and a distance between the target pixel and the reference pixel on an image. (Refer to “Bilateral Filtering for Gray and Color Images,” Proc. IEEE Int. Conf. on Computer Version, pp. 836-846 (1998). Also, a technology called a non-local means method known as noise reduction processing with higher accuracy uses block matching to calculate the similarity between the target pixel and the reference pixel. (Refer to “A non-local algorithm for image denoising,” IEEE Computer Vision and Pattern Recognition, 2005, Vol. 2, pp. 60-65 (2005)).
However, the non-local means method has an insufficient noise reduction effect due to the fact that a complete block match occurs in a case where the same region as chat for a target pixel is set as for a reference pixel.